1. Field of the invention
The present invention relates to a lamp having a layer, which is made of fluorocarbon polymer, coated on an envelope thereof, and also relates to a method for forming the layer.
2. Description of the related art
A lamp having a layer, which is made of fluorocarbon polymer, coated on a glass envelope of the lamp is known in this field. The layer is formed so as to prevent glass pieces of the glass envelope from scattering when the glass envelope of the lamp is broken. The fluorocarbon polymer is used as a material of the layer since the fluorocarbon polymer has a high melting temperature. Therefore, the layer of the fluorocarbon polymer is adapted especially to high intensity discharge lamps such as metal halide lamps whose outer envelopes have high temperature of more than 200.degree. C.
However, the conventional layer of the fluorocarbon polymer is not sufficient in its strength for high intensity discharge lamps. Therefore there is a demand to increase the strength of the layer of the fluorocarbon polymer on the outer glass envelope of the high intensity discharge lamp.
A strengthened layer of the fluorocarbon polymer is obtained by way of increasing the thickness of the layer of the fluorocarbon polymer. However, in this case the lamp has a shortcoming in that the luminous flux of the lamp emitted from the outer glass envelope of the lamp decreases, because of light absorption by the layer of the fluorocarbon polymer.
Further lamps having an improved layer of the fluorocarbon polymer are shown in the Japanese Patent Laid Open Publications No. 60-71546 and No. 64-21855. The lamp shown in the 60-71546 publication has a layer of fluorocarbon polymer containing glass fibers. But the glass fibers are mixed into the fluorocarbon polymer for increasing the adhesive strength between the layer of the fluorocarbon polymer and the outer glass envelope of the lamp, and not for increasing the strength of the layer of the fluorocarbon polymer itself, in other words not for tensile strength. The layer of the fluorocarbon polymer of this lamp is not improved in its tensile strength.
The lamp shown in the 64-21855 publication has an under layer between the layer of the fluorocarbon polymer and the outer glass envelope of the lamp. The under layer is generally called a primer layer. The under layer shown in the 64-21855 publication contains metal oxide grains dispersed therein and is coated for the same reason as the glass fibers mixed into the fluorocarbon polymer. The layer of the fluorocarbon polymer of this lamp is not improved in its strength.